Aspect Ratio Trends

A few people have pointed out to me that there is no fundamental difference between 'conventional' and 'spherical' tokamaks, and suggested that they should all be in the same table. They have said that there is a continuum of aspect ratios between about 1.1 and 10, and that it is irrelevant to separate the machines into two types.

So is it sensible or not? This is a survey of the aspect ratio of about 140 machines from the earliest days of tokamak research to the machines that are being built at present.

Looking at the aspect ratios of all 140 machines in this sample, it is clear that machines have been built across a wide range of configurations. Some machines (e.g. TCV, Lausanne) have variable configuration and cannot be easily placed on this chart.

However, it is useful to look at the trend in over the last six decades, and a pattern is seen to emerge.

Early machines with high aspect ratios.

A trend towards lower aspect ratios begins. This is not a complete sample of the machines built in that decade, but I have no reason to believe that any machines were built far outside the range indicated here.

Lower aspect ratios established as the normal configuration, with Erasmus (Belgium) leading the way (2.0).

Some machines such as ST (USA) still being built at high aspect ratio in the early part of the decade.

Evidence of the emergence of two favoured configurations, 'spherical' and 'conventional'.

The low aspect ratio 'spherical' configuration is now clearly established as a separate family, while there is continued interest in the 'conventional' configurations. Perhaps the most disappointing statistic is that fewer machines have been built in the current decade than in any other decade since the 1960s.

In conclusion, it is true that there is no fundamental difference between 'spherical' tokamaks and 'conventional' tokamaks, but there is definitely a trend towards lower aspect ratios over the decades. It is now clear that two separate configurations can be identified. A spherical tokamak community has emerged, arguing that this is the most economical route to a commercial fusion reactor.